This invention relates to electric machines and, more particularly to methods and apparatus for assembling a segmented stator for an electric machine.
Electric machines, such as motors and generators, generally include the same basic component parts. A motor housing encloses a stator and a rotor. Endbells position the rotor relative to the stator. The stator includes salient poles and slots between the salient poles. Winding wire is wound around the stator poles. A drive circuit is connected to the windings and generates a rotating magnetic field in the windings. The rotor includes rotor poles and rotates relative to the housing, the endbells and the stator due to the rotating magnetic field.
Improvements to the design of electric machines have led to more compact machines for a given torque output at reduced assembly costs. If the product is equipped with a motor that does not require additional torque, the outer dimensions of the motor and the product can be reduced without impacting performance. Alternately, the dimensions of the product can be kept the same with a motor providing increased torque and improved performance.
The torque density of machines has been improved through the use of improved magnets on the rotors, stators including segmented stator assemblies, increased slot fill, improved placement of the windings on the segmented stator, and optimized thermal dissipation. In addition to the design and material changes, the assembly processes have also improved.
Part of the assembly process for electric machines having a segmented stator involves securing stator segment assemblies to an inner wall of the housing. In the xe2x80x9chot dropxe2x80x9d process, the housing is heated and expands slightly. Expansion of the housing provides sufficient clearance to position the stator segment assemblies inside of the housing. After the housing is placed over the stator segment assemblies, the housing cools and contracts to establish an interference fit between the stator segment assemblies and the housing. In an alternate assembly process, the stator segment assemblies are press fit into the housing. Since the segmented stator typically includes several stator segment assemblies, it is difficult to hold the stator segment assemblies in position while the housing is hot dropped.
Complex assembly fixtures have been devised to locate and hold the stator segment assemblies while the housing is installed. The assembly fixture is often located on the radially outer surface of the stator segment assemblies and obstructs the placement of the housing during assembly. Other assembly techniques employ wires and/or springs to hold the individual stator segment assemblies together while the housing is installed. Oftentimes, the wire and springs fall off during this process which slows the assembly process and adversely impacts quality.
To decrease the time and cost that is required to assemble the electric machines, it is desirable to further improve the assembly process of segmented stator assemblies for electric machines.
A method of assembling a stator including a plurality of stator segment assemblies inside a housing of an electric machine according to the invention includes the step of positioning the stator segment assemblies around an outer surface of an assembly fixture. The stator segment assemblies are held against the assembly fixture using a magnetic field. The housing is then positioned over the stator segment assemblies. Thereafter, the housing with the stator secured thereto is removed from the assembly fixture.
In other features of the invention, the housing is heated to cause the housing to expand before the housing is positioned over the stator segment assemblies. Afterwards, the housing is allowed to cool and contract around the stator segment assemblies.
According to another feature of the invention, the assembly fixture has a generally cylindrical outer surface. The assembly fixture includes a magnet positioner and a plurality of permanent magnets that are held by the magnet positioner. The assembly fixture generates a magnetic field that has a radial magnetic field component that is higher than an axial magnetic field component. The higher radial magnetic field component holds the stator segments during assembly and the lower axial magnetic field component allow easy removal.
An assembly system for manufacturing a stator with a plurality of stator segment assemblies inside a housing of electric machine includes an assembly fixture that generates a magnetic field. A heater heats and expands the housing. The stator segment assemblies are positioned and held in place around the assembly fixture by the magnetic field while the heated and expanded housing is hot dropped over the stator segment assemblies. The housing cools and contracts to provide an interference fit between the stator segment assemblies and the housing.
In other features of the invention, the assembly fixture includes a cylindrical portion that is connected to a base. A sleeve is positioned on an outer surface of the cylindrical portion of the assembly fixture. The cylindrical portion includes a plurality of circular stacked laminations. Each of the stacked laminations includes a plurality of radial slots. The radial slots of the laminations are aligned and receive permanent magnets. Alternately, the assembly fixture includes an electromagnet that generates the magnetic field.
Other objects, features and advantages will be apparent from the specification, the drawings and the following claims.